1. Field of the Invention
The present invention relates to a workpiece holding device preferably for use in a polishing apparatus for producing a flat mirror surface on a workpiece such as a semiconductor wafer.
2. Description of the Prior Art
In recent years, there has been a remarkable progress in the density of integrated circuit devices leading to a trend of narrowing interline spacing, and, in the case of using optical lithography process involving less than 0.5 .mu.m line spacing particularly, the shallow depth of focus is associated to demand that the focusing plane of the stepper device be highly flat. It then becomes necessary to provide a very flat surface on semiconductor wafer, and one such method is to carry out the planarization using a polishing apparatus.
FIG. 5 shows a conventional polishing apparatus 130 having a turntable 132 with a polishing cloth 131 mounted on its top surface and a top ring 133 for holding a wafer W and pressing it onto the turntable 132. Polishing is performed by rotating the turntable 132 and the top ring 133 independently while supplying a polishing solution Q from a supply nozzle 134 on the polishing cloth so as to remove the surface material chemically as well as mechanically.
As shown in FIGS. 6 and 7, for example, the top ring 133 may be a top ring member 100, disclosed in Japanese Laid-open Patent Publication H6-198561, coupled to a pressing device and a driving device through a drive shaft 102 (spline shaft). A flange section 104 is provided at the lower end of the drive shaft 102 to extend laterally, and a number of radially extending drive pins 106 are provided on the side surface of the flange section 104. On the top surface of the top ring member 100, a corresponding number of vertically extending engaging pins 108 are provided so that the rotation of the drive shaft 102 engages the drive pins 106 with the engaging pins 108 to transmit the drive force from the drive shaft 102 to the top ring member 100.
Also, cavities 110, 112 each having a spherical inner surface, are provided respectively on the bottom end of the drive shaft 102 and on the top surface of the top ring member 100, so that a wear resistant ball 114 can be placed to form a spherical bearing device 116. This spherical bearing device 116 ensures that, even if the turntable becomes tilted with respect to the drive shaft 102, the top ring member 100 is able to follow such tilt to maintain a close contact between the workpiece and the turntable.
The polishing apparatus of such a construction performs polishing of the workpiece W, by pressing the workpiece W onto the turntable through the top ring member 100 with a certain pressing force while the top ring and the turntable are being rotated to produce a flat and mirror polished surface.
A through hole 118 is provided in the center section of the drive shaft 102, and the top section of the through hole 118 is connected to an external device such as a fluid supply device or suction device, and the bottom section of the through hole 118 is connected to one end of a tube 120 to communicate with the space or region between the top ring member 100 and the workpiece W to be polished. Various operations are performed through the tube 120, such as handling of a fluid between the external device and the polishing apparatus, vacuum holding of the wafer W on the top ring member 100 or pressing of the wafer W onto the turntable by providing a high pressure fluid during the polishing operation.
In such a conventional polishing apparatus, because the pressing force of the drive shaft 102 is designed to be transmitted through the spherical bearing 116 provided between the drive shaft 102 and the top ring member 100, a close contact of the wafer W to the turntable surface can be achieved. At the same time, however, because of the frictional forces acting between the wafer W and the polishing cloth on the turntable, the top ring member 100 is subjected to a turning moment which tends to rotate the structure 100 about the spherical bearing 116, and, depending on polishing conditions, stable polishing might not be performed.
The turning moment is proportional to the radial distance between the center of the bearing 116 and the polishing surface of the wafer W. Therefore, polishing can be made more stable, for example, by making the top ring member 100 thinner to reduce the turning moment. However, such an approach presents a problem of possible distortion of the top ring member 100 due to lowering of the stiffness thereof, as well as other problems related to shortage of space for fabricating a connection hole 122 between the tube and the inner space of the top ring member 100 or embedding the drive pins 108 in the top ring member 100, resulting in difficulties in such fabrications or reduction of the structural strength of the top ring.
Also, because the pressing force by the drive shaft 102 is transmitted through the spherical bearing 116 in the conventional polishing apparatus, the wafer W can be pressed closely against the turntable surface. However, even though the arrangement allows a high degree of freedom for tilting, because only one drive pin on the top ring side engages with one engaging pin on the drive shaft side, if the rotation of the top ring becomes unstable for any reason, the contact between the two pins becomes erratic and the transmission of rotating force becomes unstable so as to compound the rotational instability of the top ring.
Furthermore, the conventional polishing apparatus has a metallic drive shaft 102 with through hole 118 in its center section. If there are pin holes in the inner surface of the through hole 118, application of an anti-rusting coating on the surface of the through hole 118 does not provide sufficient coverage to prevent rust formation. In such a case, pure water directed to the wafer through the through hole 118, into the space between the top ring member 100 and the wafer W, becomes contaminated with rust substances, causing a danger of degrading the properties of the wafer product.